U.S. Pat. No. 3,663,355 describes a fire protected board of a flammable material on which there is placed a fire protective coating which consists of a water-soluble silicate, for example sodium or potassium silicate, which foams when subjected to high heat. This layer is then covered with a second layer consisting of a waterproof material. In the board described in this patent, the base consisting of flammable material is covered with a first layer of waterproof material which supports the foam-generating layer on which then resides the second layer of waterproof material. The waterproof material is obtained by adding a substance, for example dicalcium silicate, to the water-soluble silicate, thereby generating a self-curing mixture. The purpose of the known waterproof plate described above is to retain its fire-proof properties even under severe environmental conditions over long periods of time due to the layer construction in which the foam-generating layer is protected against environmental conditions prior to the occurrence of an actual fire.
U.S. Pat. No. 4,179,535 describes a self-hardening compound based on sodium silicates for coating substrates with a fire-retardant layer which swells up in case of a fire but solidifies again without flowing away. This compound is an aqueous solution of water-soluble silicates up to between 40 and 80% by weight as well as 20-60 weight % of hydrated silicate particles and 15-20% water. The molar ratio of SiO.sub.2 to Na.sub.2 O in both silicates should be between 2 and 3.5.
The fire resistant plate described in U.S. Pat. No. 3,663,355 affords good flame protection but it is difficult to manufacture and cannot be used in all applications due to its relatively complicated type of construction.
The protective layer described in the U.S. Pat. No. 4,179,535 has been found in tests to be impossible to produce without the addition of a protective gas (20-60% by weight) unless it includes a ratio of water glass to silicate particles of 50:50. However, such a ratio makes it difficult to apply the material, for example by means of a spatula.